1. Field of the Invention
The present invention relates generally to variable gain amplifying circuits.
2. Description of the Related Art
A variable gain amplifying circuit is used to amplify or attenuate an input signal according to a gain controlled by a gain controller. FIG. 1 illustrates a prior art variable gain amplifying circuit 100. Referring to FIG. 1, the variable gain amplifying circuit 100 includes a differential operational amplifier 12, fixed resistors R1A and R1B, and variable resistors R2A and R2B.
Referring to FIG. 1, each of the variable resistors R2A and R2B has a plurality of circuits, each having a fixed resistor and a MOSFET serving as a switch connected in series, are connected in parallel. A gain controller 14 generates several logic signals to input into the MOSFETs in the variable resistors R2A and a gain controller 16 generates several logic signals to input into the MOSFETs in the variable resistors R2B, respectively. A combined resistance of the variable resistor R2A or R2B is determined by turning on or turning off the respective MOSFETs, so that the resistances of the resistors R2A and R2B are determined. In this manner, the total gain of the variable gain amplifying circuit 100 is determined.
As known, the gain of the variable gain amplifying circuit 100 is determined according to the ratio of the feedback resistance to the input resistance. Therefore, if N gain steps are required, each of the variable resistors R2A and R2B requires N MOSFETs connected to N fixed resistors. As a result, the chip area of the variable gain amplifying circuit 100 increases as the increase of the number of N.